Non-coaxiality behaviours and unified flow rule of soil–structure interfaces

Non-coaxiality is one of the key characteristics of soil–structure interfaces that broadly occur in geotechnical engineering. A series of interface tests between gravel and a structure was performed to address and model the non-coaxiality behaviours of interfaces subjected to various three-dimensional (3D) loadings. Non-coaxiality of the interface was identified during the yielding phase when subjected to 3D cycling of shear stress and could be captured by the non-coaxiality angle. The non-coaxiality angle does not evolve as a result of 3D cyclic shearing but depends primarily on the magnitude and direction of the current shear stress and the direction of the shear stress increment. The shear stress amplitude ratio, shear stress amplitude, shear stress rotation and initial shear stress significantly affect the magnitude and change pattern of the non-coaxiality angle, attributed to varied shear stress and shear stress increment vectors. A unified flow rule for interfaces was established based on 3D interface test analysis. The flow rule perfectly determines the flow direction of different kinds of interfaces prior to and at the mobilisation of shear strength under diverse two-dimensional and 3D loading conditions, such as arbitrary shear paths and differing shear stress amplitudes, shear stress amplitude ratios, initial shear stresses, normal stresses, shear strengths and anisotropy characteristics.